With only two wheels, they have to be placed perpendicular to the tracks so it's literally a tricycle now. But how does the stylus know when to stop moving forward or overshooting.
Skating
Hi 2wice,
Your proposed geometry with either two or three curves has a virtual pivot somewhere to the left of the the tonearm's center line. That virtual pivot is not located in a fixed position, but wanders about as the radii of the curves become smaller. The fact that the direction of the drag force passes the virtual pivot on its right side, causes the tonearm to skate! The fact, that the drag force is governed by, a) the vinyl composition, b) the tracking force and c) the frequency content of the LP, makes it unpredictable and it will have to be controlled by some type of servo. I know this because my 2010 tonearm, functioning basically like yours, skated and it took me until two days ago to perfect it. A kind gentleman on the internet designed the electronic servo for me and now it works! It is a passive servo that provides just enough of an opposing force to the skating force to allow it to track with 100% accuracy.
I am not trying to discourage you, but just to warn you, as to what to expect.
Happy experimenting,
Ralf
Hi 2wice,
Your proposed geometry with either two or three curves has a virtual pivot somewhere to the left of the the tonearm's center line. That virtual pivot is not located in a fixed position, but wanders about as the radii of the curves become smaller. The fact that the direction of the drag force passes the virtual pivot on its right side, causes the tonearm to skate! The fact, that the drag force is governed by, a) the vinyl composition, b) the tracking force and c) the frequency content of the LP, makes it unpredictable and it will have to be controlled by some type of servo. I know this because my 2010 tonearm, functioning basically like yours, skated and it took me until two days ago to perfect it. A kind gentleman on the internet designed the electronic servo for me and now it works! It is a passive servo that provides just enough of an opposing force to the skating force to allow it to track with 100% accuracy.
I am not trying to discourage you, but just to warn you, as to what to expect.
Happy experimenting,
Ralf
Removing the 3rd curve and the fixed point associated with is a problem for the overshoot, but the 3rd point can be placed on either of the 2 remaining curves.
As long as it forms a rigid triangle that cannot flex and that it cannot loose contact from the curve as it moves.
Imagine something similar to the 3 wheels of the curved rail you posted before, but with one or another point on the second curve.
I'm still doing the modeling for that scenario, but want to complete the maths first.
Hi Ralf
I'm hoping you can clear this up for me as I can admit the complexity of skating force is a bit muddled to me atm.
As I understand, skating force is a function of frictional force tangent to the groove + a whole bunch of other variables that result in a force perpendicular to the groove.
This skating force acts on the stylus causing the arm to deflect towards the center of the record.
I hope I got this right.
But what if the rotation/travel of the arm is constrained by the position of the stylus in the groove?
Would skating force still be valid and what would the effects be on the stylus in this case?
Cheers
Trenton
Skating
Hi Trenton,
You have it right.
If the skating force was moderate, The stylus would bear on the inner flank of the groove with greater force than the outer flank of the groove. As the skating force increases, the stylus would be deflected to the point, where it would jump the groove. Assuming that you have a turn table, and maybe a test record and further assuming that the test record has a band without grooves, you can see skating in action by lowering your tonearm's stylus onto that grooveless band and observe the tonearm as it skates toward the label area. That is true for all tonearms with offset head shells and tonearms with virtual pivots to the left of the tonearm, such as yours and mine.
Any response from you will not be answered until at least 8 hours from now, as I am going to bed (yawn).
Hi Trenton,
You have it right.
If the skating force was moderate, The stylus would bear on the inner flank of the groove with greater force than the outer flank of the groove. As the skating force increases, the stylus would be deflected to the point, where it would jump the groove. Assuming that you have a turn table, and maybe a test record and further assuming that the test record has a band without grooves, you can see skating in action by lowering your tonearm's stylus onto that grooveless band and observe the tonearm as it skates toward the label area. That is true for all tonearms with offset head shells and tonearms with virtual pivots to the left of the tonearm, such as yours and mine.
Any response from you will not be answered until at least 8 hours from now, as I am going to bed (yawn).
Thanks very much, but what about the arm being constrained by the position of the stylus in the groove? If I understand it correctly the force acts on the arm through the stylus. If the arm cannot be forced off the constraints the skating force can be ignored.
And if that is the case what happens to the force generated?
And if that is the case what happens to the force generated?
virtual pivot
Thank you so much for the explanation, Ralf. I think now I got a grasp of this "virtual pivot," something I never thought existed!
My question is that IF this virtual pivot is indeed a fixed position, will there be skating force towards the spindle? I am thinking of a split-plane design a la Dynavector but both planes form a straight line with linear motion bearings to move the carriage that holds the main armwand. Essentially a Thales geometry that allows the arm to lengthen as it travels toward the spindle. This way, I hope, every position of the stylus is on a 'neutral" line. Obviously a guiding mechanism is needed to prevent it from overshooting forward. I hope I describe it properly.
Thank you so much for the explanation, Ralf. I think now I got a grasp of this "virtual pivot," something I never thought existed!
My question is that IF this virtual pivot is indeed a fixed position, will there be skating force towards the spindle? I am thinking of a split-plane design a la Dynavector but both planes form a straight line with linear motion bearings to move the carriage that holds the main armwand. Essentially a Thales geometry that allows the arm to lengthen as it travels toward the spindle. This way, I hope, every position of the stylus is on a 'neutral" line. Obviously a guiding mechanism is needed to prevent it from overshooting forward. I hope I describe it properly.
Last edited:
Hi Ralf
I've gone through every text I could find on the subject since the 20's and I can find nothing that indicates skate force on virtual pivots, fixed or moving. The major cause is the reaction force (force vector) of a fixed pivot as a result of the offset and the overhang as well as groove modulation, speed etc.
There is no fixed pivot for the force vector to terminate on as well as there being zero overhang as well as the force vector=tangent vector.
No skate force?
If you know of any text that could show otherwise I would appreciate it if you could point me in the right direction.
I've gone through every text I could find on the subject since the 20's and I can find nothing that indicates skate force on virtual pivots, fixed or moving. The major cause is the reaction force (force vector) of a fixed pivot as a result of the offset and the overhang as well as groove modulation, speed etc.
There is no fixed pivot for the force vector to terminate on as well as there being zero overhang as well as the force vector=tangent vector.
No skate force?
If you know of any text that could show otherwise I would appreciate it if you could point me in the right direction.
Ja thought so thanks, then there won't be any skate force on my geometry.
Just when I though I wasn't confused anymore.
If the string is fixed to the stylus and you pull the string in which direction will make it skate?
The friction force is tangential is it not?
alighiszem,
The test you describe is correct, but you are a latecomer crashing the party with this one. If everyone would read my post #949, I proposed this exact test a year ago on this thread.
In post #949 I said:
See post #949:
http://www.diyaudio.com/forums/analogue-source/165878-angling-90-tangential-pivot-tonearms-95.html
Ray K
Litmus test for skating
2wice,
Please see my litmus test for skating in post #949:
http://www.diyaudio.com/forums/analogue-source/165878-angling-90-tangential-pivot-tonearms-95.html
The string test is unambiguous and separates theory from fact. As a practical matter, the string should be attached to the headshell, not the stylus. We don't want to damage the stylus and besides, it would be hard to tie a knot that small! Pulling the string in line with the headshell, i.e. tangentially, simulates the stylus friction force. Yes, the simulated 'friction force' is tangential, but if the tangential string force vector is offset or not in line with the pivot, be it fixed or virtual, the tangential friction force will resolve itself into some rotary component about the pivot point, and this manifests itself as skating. The various types of 'multiple linkage' arms might have a fixed pivot (like Garrard Zero 100) or a motion that behaves as a virtual pivot which may move (like a carriage moving on curved rails like yours).
It can be difficult to visualize the forces on a complex linkage, but the litmus test is definitive. Pull on the string. If the headshell moves sideways, then the arm will skate.
Ray K
Just when I though I wasn't confused anymore.
If the string is fixed to the stylus and you pull the string in which direction will make it skate?
The friction force is tangential is it not?
2wice,
Please see my litmus test for skating in post #949:
http://www.diyaudio.com/forums/analogue-source/165878-angling-90-tangential-pivot-tonearms-95.html
The string test is unambiguous and separates theory from fact. As a practical matter, the string should be attached to the headshell, not the stylus. We don't want to damage the stylus and besides, it would be hard to tie a knot that small! Pulling the string in line with the headshell, i.e. tangentially, simulates the stylus friction force. Yes, the simulated 'friction force' is tangential, but if the tangential string force vector is offset or not in line with the pivot, be it fixed or virtual, the tangential friction force will resolve itself into some rotary component about the pivot point, and this manifests itself as skating. The various types of 'multiple linkage' arms might have a fixed pivot (like Garrard Zero 100) or a motion that behaves as a virtual pivot which may move (like a carriage moving on curved rails like yours).
It can be difficult to visualize the forces on a complex linkage, but the litmus test is definitive. Pull on the string. If the headshell moves sideways, then the arm will skate.
Ray K
Thanks, I get that now.
I was hoping for a clearer theoretical explanation, but a physical test will. do.
Like this string shown in red and direction ?
Regards
